I’m reading a book in which these two words often occur right next to each other as so: “… dass das …” Is there a difference in their pronunciation? If so, how can I make that clear? Do I need to use a certain pronunciation to differentiate it from a phrase like “… das das …” for example or is the context sufficient for that purpose?
The unmarked and semi-clear pronunciation of both words is identical. In spite of das having only a single letter, the vowel is short which is common in one-syllable words (compare es, des which also have a short vowel and where the vowel length is phonetic). Which one is which is usually differed grammatically by the type of sentence they are in. So far the theory.
In practice, some additional effects come into play:
Speakers have a tendency to reduce the vowel in das into something closer to des (with a shwa). Bavarians (and probably other southerners, too) may even reduce it to ’s. Dass is hardly ever subject to reduction.
When dictating, there can be a tendency to overstress the length of das’ a making it sound like daas.